[jdom-interest] Documents are now serializable

[Jason Hunter]

FYI, JDOM Documents are now fully and correctly serializable.  (At least
I think they are -- my tests passed.)

The hold out for this feature has been the Namespace class because
instances must be created using the factory model so it wasn't enough
for Namespace to just implement Serializable.  Tonight I added the right
hooks so serialization can occur while still adhering to the factory
model.  See jdom-commits for details.  CVS has the latest and greatest.

-jh-